Granular-Oxide Steel Sheets for Hydrogen and Zinc Penetration Control
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Solution Overview
Problem
High strength steel sheets used in corrosive environments suffer from hydrogen embrittlement cracking and liquid metal embrittlement due to penetration of hydrogen and zinc from plating, which degrade the steel's properties, and existing methods do not effectively control the form of internal oxidation layers to prevent these issues.
Innovation Solution
Forming granular oxides inside the steel sheet with controlled size and density, and an Si-Mn depleted layer to trap hydrogen and zinc, promoting their diffusion and discharge, while ensuring sufficient interdiffusion for plating adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If internal oxidation layer is formed to improve hydrogen embrittlement resistance, then hydrogen penetration is reduced, but plating adhesion deteriorates due to oxide film formation on surface
Solution Approach 1:
The patent applies preliminary action by forming the internal oxidation layer during the steelmaking or rolling process before plating is applied. This pre-formed internal oxide structure serves as a hydrogen barrier that is already in place when the steel sheet enters the plating process, allowing subsequent plating to proceed effectively without being blocked by surface oxides.
Solution Approach 2:
The patent implements local quality by creating an internal oxidation layer with specific characteristics (oxide type, distribution, and depth) that differs from the surface layer. The internal oxides are positioned at controlled depths (e.g., 5-50 μm from surface) and have different compositions than surface oxides, providing hydrogen barrier functionality while maintaining surface quality suitable for plating adhesion.
2Weight of moving object
If high strength steel sheet is used to reduce vehicle body weight, then fuel economy improves, but hydrogen embrittlement cracking occurs in corrosive environments
Solution Approach 1:
The patent converts the harmful effect of internal oxides (which typically deteriorate plating adhesion) into a beneficial hydrogen barrier. By controlling the internal oxide characteristics, the patent transforms what would normally be a defect into a functional feature that prevents hydrogen embrittlement while maintaining acceptable plating quality.
Solution Approach 2:
The patent applies parameter changes by precisely controlling multiple parameters of the internal oxidation layer including oxide composition (SiO2, MnO, FeO ratios), depth distribution (5-50 μm from surface), particle size, and concentration. These parameter optimizations enable the internal oxides to function as effective hydrogen barriers without compromising the steel's overall performance or plating adhesion.
3Manufacturing precision
If plating layer is applied on steel sheet with internal oxidation layer, then plateability improves compared to external oxidation, but zinc penetration during hot stamping causes liquid metal embrittlement
Solution Approach 1:
The patent applies preliminary action by pre-forming the internal oxidation layer with specific characteristics before the hot stamping process. This internal oxide structure is positioned and configured in advance to serve as a barrier against zinc penetration during subsequent high-temperature processing, preventing liquid metal embrittlement before it can occur.
Solution Approach 2:
The patent uses the internal oxidation layer as an intermediary barrier between the plating layer and the steel base metal. During hot stamping, this internal oxide layer intercepts and blocks zinc atoms attempting to penetrate into the steel, preventing direct contact between molten zinc and the steel matrix, thus avoiding liquid metal embrittlement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly improves hydrogen embrittlement and liquid metal embrittlement resistance, while maintaining high plateability by controlling the form and distribution of oxides, enhancing the steel's performance in corrosive environments.
Implementation Method 1
granular oxides present inside a crystal grain of the metallographic structure... trap hydrogen and zinc
Implementation Method 2
granular oxides... function as trap sites for Zn which could penetrate the steel during hot stamping or welding
Implementation Method 3
hydrogen generated in the process of corrosion will penetrate into the steel. The hydrogen penetrating the steel will segregate at the martensite grain boundaries
Implementation Method 4
the Zn included in the plating layer has melted. In this case, the molten Zn will sometimes penetrate into the steel and cause cracks inside the steel sheet
Data Source
Figure 1~3
Figure 4~5
AI summary
A steel sheet comprising, by mass%, C: 0.05 to 0.40%, Si: 0.2 to 3.0%, and Mn: 0.1 to 5.0%, wherein a surface layer of the steel sheet contains granular oxides, an average grain size of the granular oxides is 300 nm or less, a number density of granular oxides is 4.0/µm2 or more, the steel sheet comprises an Si-Mn depleted layer having a thickness of 3.0 µm or more from the surface of the steel sheet, and Si and Mn contents of the Si-Mn depleted layer not containing oxides at the 1/2 position of the thickness are respectively less than 10% of the Si and Mn contents at the sheet thickness center part of the steel sheet, and a plated steel sheet using the same are provided.